This invention relates to an objective lens for use in an optical pickup device which is capable of carrying out the reproducing and/or recording of optical disks having different thicknesses of the transparent substrate respectively and uses light sources having different wavelengths respectively and an optical pickup device employing said lens.
In recent years, accompanied by the practical use of the red semiconductor laser having a short wavelength, it has been advanced the development of a high-density optical disk DVD (digital video disk) which has about the same size as a conventional optical disk CD (compact disk) and has been made to have a large capacity. In an optical pickup device for carrying out recording and/or reproducing of information for this DVD, in the case where a semiconductor laser having a short wavelength of 635 nm is used for the light source, the numerical aperture NA of the objective lens at the side of the optical disk for converging the above-mentioned laser beam is made approximately 0.6. Further, in addition to the CD and DVD, optical disks of various standards, for example, a CD-R (a direct read after write, writing once compact disk), an MD (a mini-disk), etc. are commercialized and have come into general use. In Table 1, the thickness of the transparent substrate and the required numerical aperture of various optical disks are shown.
Further, in an optical pickup device for carrying out the recording and/or reproducing for a CD-R, it is necessary that the wavelength of the light source xcex is equal to 780 nm; however, for other optical disks, it is possible to use a light source having a wavelength other than the wavelengths noted in Table 1, and in such cases, the required numerical aperture NA is made to be varied in accordance with the light source wavelength xcex used. For example, the required numerical aperture NA is approximately expressed by NA=xcex(xcexcm)/1.73 for a CD, and by NA=xcex(xcexcm)/1.06 for a DVD. The numerical aperture (required numerical aperture) referred to in this specification means the numerical aperture of a converging optical system as seen from the side of the optical disk, and the required numerical aperture means the numerical aperture calculated from the spot size d which is required on the recording surface of the optical disk and the used wavelength xcex, to be expressed generally by NA=0.83xc3x97xcex÷d.
As described in the above, there are various optical disks which are different in the thickness of the transparent substrate, the recording density, and the used wavelength on the market; however, it is a large burden for a user to purchase an apparatus for recording and/or reproducing information capable of recording and/or reproducing information to be exclusively used for each of the optical disks. Therefore, it has been proposed an apparatus for recording and/or reproducing information equipped with an interchangeable optical pickup device capable of coping with various optical disks.
For such an optical pickup device, one that is provided with converging optical systems corresponding to different optical disks respectively, and is designed to switch over the optical systems in accordance with the optical disk to reproduce has been proposed. However, in this optical pickup device, a plurality of converging optical systems are required which makes the structure complicated, resulting in making the cost high, which is not desirable.
Therefore, it has been proposed an optical pickup device which reproduces a plurality of optical disks using a single converging optical system. For example, in the publication of the unexamined patent application H7-302437, it is disclosed an optical pickup device which carries out reproducing by dividing the refracting surface into a plurality of ring-shaped bands and making each of the divisional surfaces converge the beam on one of the optical disks having different thicknesses respectively.
However, the optical pickup device disclosed in the publication of the unexamined patent application H7-302437 has the problem that it requires a large laser output because the single objective lens has more than two focal points simultaneously which reduces the spot light quantity per each focal point.
On the other hand, the applicant has proposed the following optical pickup device in the publication of the unexamined patent application H11-96585. That is, the optical pickup device comprises an objective lens provided with a plurality of ring-shaped bands obtained through dividing the lens surface by concentric circles, wherein, for a plurality of light sources having different wavelengths respectively and plural transparent substrates having different thicknesses respectively, each of the ring-shaped bands has its aberration corrected to the refraction limit or under for each of the optical disks by positively utilizing the spherical aberration produced by them. This objective lens has a function that the required numerical aperture can be automatically obtained in accordance with the used wavelength and/or the thickness of the transparent substrate.
However, in the case where the correction is made to the refraction limit or under, it is more desirable to take it into consideration the intensity distribution in the point image, that is, the central intensity of the spot light, in addition to the spot size (spreading of the point image). In order to make this central intensity higher, it is necessary to remark-the phase of the light flux (deviation of the wave front aberration), but there is no description concerning this deviation of the wave front aberration with respect to the optical pickup device disclosed in the publication of unexamined patent application H11-96585.
It is an object of this invention to provide an optical pickup device comprising an objective lens which has four or more divisional surfaces formed and is capable of correcting the deviation of the wave front aberration in the divisional surfaces inside the required numerical aperture for each of the optical disks and the objective lens. Further, it is another object of this invention to provide an optical pickup device and an objective lens having a small amount of loss of light quantity and an excellent S-letter characteristic.
The above-mentioned objects are accomplished by any one of the following structures.
(1-1) An optical pickup device for conducting reproducing and/or recording of information in at least two kinds of optical information recording media, comprises
a first light source to emit light flux having a first wavelength (xcex1) in order to conduct reproducing and/or recording of the information in a first optical information recording medium having a first transparent substrate;
a second light source to emit light flux having a second wavelength (xcex2) longer than the first wavelength in order to conduct reproducing and/or recording of the information of a second optical information recording medium having a second transparent substrate;
a converging optical system for converging the light flux having the first wavelength or the light flux having the second wavelength on the first information recording surface of the first optical information recording medium or on the second information recording surface of the second optical information recording medium; and
an optical detector for receiving and detecting the light flux reflected by the first information recording surface or by the second information recording surface,
wherein a required numerical aperture for the first optical information recording medium is larger than that for the second optical information recording medium;
wherein the converging optical system comprises at least one optical element and the optical element has ring-shaped divided surfaces partitioned by stepped portions on at least one surface for dividing the light flux incident on the optical element into k (kxe2x89xa74) pieces of ring-shaped light flux in which light flux nearest to the optical axis of the optical element is regarded as the first light flux and other light flux are called consecutively the second, the third - - - , and the k-th light flux toward the outer side in the direction perpendicular to the optical axis;
wherein when conducting reproducing or recording of the first optical information recording medium, the first light flux and the k-th light flux among the light flux of the first wavelength having passed the ring-shaped divided surfaces are converged on the first information recording surface in order to conduct reproducing or recording of the information of the first optical information recording medium, a spherical aberration component of a wave front aberration is made 0.05xcex1 rms or less at a position of the best image plane, and at least two pieces of light flux among the light flux of the first wavelength from the second light flux to the (kxe2x88x921)th light flux form an apparent best image plane at a position different from the position of the best image plane formed by the first light flux and the k-th light flux; and
wherein at the position of the best image plane of the first light flux and the k-th light flux, a wave front aberration of a ray in each light flux of the firs wavelength from the first light flux to the k-th light flux passing in a required numerical aperture for the first optical information recording medium is made approximately mixcex1, where mi is an integer and i=1, 2, - - - , k.
(1-2) In the optical pickup device set forth in (1-1), when conducting reproducing or recording of the first optical information recording medium, at the position of the best image plane of the first light flux and k-th light flux, an amount of the wave front aberration (w1) of the ray in each light flux of the first wavelength from the first light flux to the kth light flux passing in the required numerical aperture for the first optical information recording medium satisfies the following conditional formula:
(mixe2x88x920.30)xcex1xe2x89xa6w1xe2x89xa6(mi+0.30)xcex1 
(1-3) In the optical pickup device set forth in (1-1), when conducting reproducing or recording of the second optical information recording medium, the first light flux among the light flux of the second wavelength having passed the ring-shaped divided surfaces is converged on the second information recording surface in order to conduct reproducing or recording of the information of the second optical information recording medium, a spherical aberration component of a wave front aberration is made 0.07xcex2 rms or less at a position of the best image plane, and at the position of the best image plane of the first light flux, a wave front aberration of a ray in each light flux of the second wavelength from the first light flux to the (kxe2x88x921)th light flux passing in a required numerical aperture for the second optical information recording medium is made approximately nixcex2, where ni is an integer and i=1, 2, - - - , kxe2x88x921.
(1-4) In the optical pickup device set forth in (1-3), when conducting reproducing or recording of the second optical information recording medium, at the position of the best image plane of the first light flux, an amount of a wave front aberration (w2) of a ray in each light flux of the second wavelength from the first light flux to (kxe2x88x921)th light flux passing in a required numerical aperture for the second optical information recording medium satisfies the following conditional formula:
(nixe2x88x920.30)xcex2xe2x89xa6w2xe2x89xa6(ni+0.30)xcex2 
(1-5) In the optical pickup device set forth in (1-3), the difference between the maximum value of the integer mi and the minimum value of the integer mi and the difference between the maximum value of the integer ni and the minimum value of the integer ni both are 2 or more.
(1-6) In the optical pickup device set forth in (1-1), k is 4 and the following conditional formulas are satisfied:
0.62xe2x89xa6h1/0.5xcfx86maxxe2x89xa60.69 
0.70 less than h3/0.5xcfx86maxxe2x89xa60.80 
where xcfx86max denotes a diameter of an aperture stop when conducting reproducing or recording of the first optical information recording medium, hi denotes a height from a boundary of the ring-shaped stepped portion to divide the light flux of the first wavelength into the first light flux and the second light flux to the optical axis, and h3 denotes a height from a boundary of the ring-shaped stepped portion to divide the light flux of the first wavelength into the third light flux and the fourth light flux to the optical axis.
(1-7) In the optical pickup device set forth in (1-1), k is 6, and when conducting reproducing or recording of the first optical information recording medium, the second light flux and the fifth light flux in the light flux of the first wavelength form an apparent best image plane at a position different from the position of the best image plane formed by the sixth light flux.
(1-8) In the optical pickup device set forth in (1-1), k is 6 and the following conditional formulas are satisfied:
0.52xe2x89xa6h1/0.5xcfx86maxxe2x89xa60.67 
0.70 less than h5/0.5xcfx86maxxe2x89xa60.82 
where xcfx86max denotes a diameter of an aperture stop when conducting reproducing or recording of the first optical information recording medium, h1 denotes a height from a boundary of the ring-shaped stepped portion to divide the light flux of the first wavelength into the first light flux and the second light flux to the optical axis, and h5 denotes a height from a boundary of the ring-shaped stepped portion to divide the light flux of the first wavelength into the fifth light flux and the sixth light flux to the optical axis.
(1-9) In the optical pickup device set forth in (1-1), the optical element is an objective lens having an aspherical surface.
(1-10) In the optical pickup device set forth in (1-1), when conducting reproducing or recording of the first optical information recording medium, in the ring-shaped divided surfaces to pass at least two pieces of light flux among the first light flux of the first wavelength from the second light flux to the (kxe2x88x921)th light flux to form an apparent best image plane at a position different from that of the best image plane formed by the first light flux and the k-th light flux, a depth of the stepped portion of the ring-shaped divided surfaces at the optical axis side is different from a depth of the stepped portion at a side reverse to the optical axis side.
(1-11) In the optical pickup device set forth in (1-9), when conducting reproducing or recording of the first optical information recording medium, in the ring-shaped divided surfaces to pass at least two pieces of light flux among the first light flux of the first wavelength from the second light flux to the (kxe2x88x921)th light flux to form an apparent best image plane at a position different from that of the best image plane formed by the first light flux and the k-th light flux, an aspherical shape of the ring-shaped divided surfaces is different from that of the other ring-shaped divided surfaces.
(1-12) An objective lens for use in an optical pickup device for conducting reproducing and/or recording of information of at least two kinds of optical information recording media, comprising:
ring-shaped divided surfaces partitioned by stepped portions provided on at least one surfaces of the objective lens,
wherein the ring-shaped divided surfaces divide light flux incident on the objective lens into k (kxe2x89xa74) pieces of ring-shaped light flux in which light flux nearest to the optical axis of the optical element is regarded as the first light flux and other light flux are called consecutively the second, the third - - - , and the k-th light flux toward the outer side in the direction perpendicular to the optical axis;
wherein when conducting reproducing or recording of a first optical information recording medium having a first transparent substrate, among a first light flux of a wavelength (xcex1) having passed the ring-shaped divided surfaces, the first light flux and the k-th light flux are converged on a first information recording surface so as to conduct reproducing or recording of information of the first optical information recording medium, a spherical aberration component of a wave front aberration is made 0.05xcex1 rms or less at a position of the best image plane, and at least two pieces of light flux among the light flux of the first wavelength from the second light flux to the (kxe2x88x921)th light flux form an apparent best image plane at a position different from the position of the best image plane formed by the first light flux and the k-th light flux; and
wherein at the position of the best image plane of the first light flux and the k-th light flux, a wave front aberration of a ray in each light flux of the firs wavelength from the first light flux to the k-th light flux passing in a required numerical aperture for the first optical information recording medium is made approximately mixcex1, where mi is an integer and i=1, 2, - - - , k.
(1-13) In the objective lens set forth in (1-12), when conducting reproducing or recording of the first optical information recording medium, at the position of the best image plane of the first light flux and k-th light flux, an amount of the wave front aberration (w1) of the ray in each light flux of the first wavelength from the first light flux to the k-th light flux passing in the required numerical aperture for the first optical information recording medium satisfies the following conditional formula:
(mixe2x88x920.30)xcex1xe2x89xa6w1xe2x89xa6(mi+0.30)xcex1 
(1-14) In the objective lens set forth in (1-12), when conducting reproducing or recording of the second optical information recording medium having a second transparent substrate thicker than the first transparent substrate, the first light flux among the light flux of the second wavelength having passed the ring-shaped divided surfaces is converged on the second information recording surface in order to conduct reproducing or recording of the information of the second optical information recording medium, a spherical aberration component of a wave front aberration is made 0.07xcex2 rms or less at a position of the best image plane, and at the position of the best image plane of the first light flux, a wave front aberration of a ray in each light flux of the second wavelength from the first light flux to the (kxe2x88x921)th light flux passing in a required numerical aperture for the second optical information recording medium is made approximately nixcex2, where ni is an integer and i=1, 2, - - - , kxe2x88x921.
(1-15) In the objective lens set forth in (1-14), when conducting reproducing or recording of the second optical information recording medium, at the position of the best image plane of the first light flux, an amount of a wave front aberration (w2) of a ray in each light flux of the second wavelength from the first light flux to (kxe2x88x921)th light flux passing in a required numerical aperture for the second optical information recording medium satisfies the following conditional formula:
(nixe2x88x920.30)xcex2xe2x89xa6w2xe2x89xa6(ni+0.30)xcex2 
(1-16) In the objective lens set forth in (1-14), the difference between the maximum value of the integer mi and the minimum value of the integer mi and the difference between the maximum value of the integer ni and the minimum value of the integer ni both are 2 or more.
(1-17) In the objective lens set forth in (1-12), k is 4 and the following conditional formulas are satisfied:
0.62xe2x89xa6h1/0.5xcfx86maxxe2x89xa60.69 
0.70 less than h3/0.5xcfx86maxxe2x89xa60.80 
where xcfx86max denotes a diameter of an aperture stop when conducting reproducing or recording of the first optical information recording medium, h1 denotes a height from a boundary of the ring-shaped stepped portion to divide the light flux of the first wavelength into the first light flux and the second light flux to the optical axis, and h3 denotes a height from a boundary of the ring-shaped stepped portion to divide the light flux of the first wavelength into the third light flux and the fourth light flux to the optical axis.
(1-18) In the objective lens set forth in (1-12), k is 6, and when conducting reproducing or recording of the first optical information recording medium, the second light flux and the fifth light flux in the light flux of the first wavelength form an apparent best image plane at a position different from the position of the best image plane formed by the sixth light flux.
(1-19) In the objective lens set forth in (1-12), k is 6 and the following conditional formulas are satisfied:
0.52xe2x89xa6h1/0.5xcfx86maxxe2x89xa60.67 
0.70xe2x89xa6h5/0.5xcfx86maxxe2x89xa60.82 
where xcfx86max denotes a diameter of an aperture stop when conducting reproducing or recording of the first optical information recording medium, h1 denotes a height from a boundary of the ring-shaped stepped portion to divide the light flux of the first wavelength into the first light flux and the second light flux to the optical axis, and h5 denotes a height from a boundary of the ring-shaped stepped portion to divide the light flux of the first wavelength into the fifth light flux and the sixth light flux to the optical axis.
(1-20) In the objective lens set forth in (1-12), when conducting reproducing or recording of the first optical information recording medium, in the ring-shaped divided surfaces to pass at least two pieces of light flux among the first light flux of the first wavelength from the second light flux to the (kxe2x88x921)th light flux to form an apparent best image plane at a position different from that of the best image plane formed by the first light flux and the k-th light flux, a depth of the stepped portion of the ring-shaped divided surfaces at the optical axis side is different from a depth of the stepped portion at a side reverse to the optical axis side.
(1-21) In the objective lens set forth in (1-12), when conducting reproducing or recording of the first optical information recording medium, in the ring-shaped divided surfaces to pass at least two pieces of light flux among the first light flux of the first wavelength from the second light flux to the (kxe2x88x921)th light flux to form an apparent best image plane at a position different from that of the best image plane formed by the first light flux and the k-th light flux, an aspherical shape of the ring-shaped divided surfaces is different from that of the other ring-shaped divided surfaces.
(1-22) An optical information recording media reproducing or recording apparatus for conducting reproducing and/or recording of information in at least two kinds of optical information recording media, comprising:
an optical pickup device comprising
a first light source to emit light flux having a first wavelength (xcex1) in order to conduct reproducing and/or recording of the information in a first optical information recording medium having a first transparent substrate;
a second light source to emit light flux having a second wavelength (xcex2) longer than the first wavelength in order to conduct reproducing and/or recording of the information of a second optical information recording medium having a second transparent substrate;
a converging optical system for converging the light flux having the first wavelength or the light flux having the second wavelength on the first information recording surface of the first optical information recording medium or on the second information recording surface of the second optical information recording medium; and
an optical detector for receiving and detecting the light flux reflected by the first information recording surface or by the second information recording surface,
wherein a required numerical aperture for the first optical information recording medium is larger than that for the second optical information recording medium;
wherein the optical pickup device comprises at least one optical element and the optical element has ring-shaped divided surfaces partitioned by stepped portions on at least one surface for dividing the light flux incident on the optical element into k (k greater than 4) pieces of ring-shaped light flux in which light flux nearest to the optical axis of the optical element is regarded as the first light flux and other light flux are called consecutively the second, the third - - - , and the k-th light flux toward the outer side in the direction perpendicular to the optical axis;
wherein when conducting reproducing or recording of the first optical information recording medium, the first light flux and the k-th light flux among the light flux of the first wavelength having passed the ring-shaped divided surfaces are converged on the first information recording surface in order to conduct reproducing or recording of the information of the first optical information recording medium, a spherical aberration component of a wave front aberration is made 0.05xcex1 rms or less at a position of the best image plane, and at least two pieces of light flux among the light flux of the first wavelength from the second light flux to the (kxe2x88x921)th light flux form an apparent best image plane at a position different from the position of the best image plane formed by the first light flux and the k-th light flux; and
wherein at the position of the best image plane of the first light flux and the k-th light flux, a wave front aberration of a ray in each light flux of the firs wavelength from the first light flux to the k-th light flux passing in a required numerical aperture for the first optical information recording medium is made approximately mixcex, where mi is an integer and i=1, 2, - - - , k.
Further, the above-mentioned objects are accomplished also by any one of the following desirable structures.
(2-1) An objective lens for use in an optical pickup device which comprises a converging optical system including an objective lens for converging light flux from light sources having different wavelengths respectively on the recording surface of an optical disk and a light receiving means for detecting reflected light from said recording surface and is capable of recording and/or reproducing information for a first and a second optical disks having transparent substrates of different thicknesses respectively (wherein the required numerical aperture of the objective lens for the first optical disk is supposed to be larger than that for the second optical disk),
wherein ring-shaped divisional surfaces for dividing the incident light flux by refracting action into k (kxe2x89xa74) pieces of ring-shaped light flux (where the light flux are called consecutively the first, the second, - - - , and the k-th light flux from the optical axis side toward the outer side) are formed on at least one of the surfaces of said objective lens, when said first optical disk is used, the spherical aberration component of the wave front aberration of said first and k-th light flux at the position of the best image plane formed by said first and k-th light flux is 0.05xcex1 rms or under (xcex1: the wavelength of the light source for the first optical disk),
for at least two pieces of light flux among said second to (kxe2x88x921)th light flux, the apparent position of the best image plane comes to a position different from said position of the best image plane formed by said first and k-th light flux, and
at said position of the best image plane formed by said first and k-th light flux, the wave front aberration of the rays in each of said first to k-th light flux passing the inside of the required numerical aperture for said first optical disk becomes approximately mixcex1 (mi is an integer, where i=1, 2, - - - , k).
According to the objective lens described in the structure (2-1), because it is designed to make the residual aberration small for the thickness of the substrate falling within the range from the substrate thickness of said first optical disk to the substrate thickness of said second optical disk-the recording and/or reproducing of information can be suitably performed for plural kinds of optical disks.
(2-2) When the aforesaid first optical disk is used, at the aforesaid position of the best image plane formed by the aforesaid first and k-th light flux, the amount of the wave front aberration w1 of the rays in each of said first to k-th light flux satisfy the following condition:
(mixe2x88x920.30)xcex1xe2x89xa6w1xe2x89xa6(mi+0.30)xcex1xe2x80x83xe2x80x83(1). 
This is because, for example, in the case where a DVD is used for the optical disk, if w1 does not satisfy the above expression (1), deviation of the wave front aberration is produced, which lowers the spot light intensity.
(2-3) When carrying out the recording and/or reproducing of information by using a light flux from the light source having a wavelength of xcex2 for the aforesaid second optical disk-the wave front aberration of the rays in the first to (k-1)th light flux becomes approximately nixcex2 (ni is an integer, where i=1, 2, - - - , kxe2x88x921). Accordingly, in said second optical disk-the residual aberration can be made small, and owing to it, the recording and/or reproducing of information can be carried out for said second optical disk.
(2-4) When carrying out the recording and/or reproducing of information by using a light flux from the light source having a wavelength of xcex2 for the aforesaid second optical disk-the amount of the wave front aberration w2 of the rays in each of the said first to (kxe2x88x921)th light flux satisfy the following condition in the required numerical number for said second optical disk:
(mixe2x88x920.30)xcex2xe2x89xa6w2xe2x89xa6(mi+0.30)xcex2xe2x80x83xe2x80x83(2). 
For example, in the case where a CD is used for the optical disk, if w2 does not satisfy the above expression (2), deviation of the wave front aberration is produced, which lowers the spot light intensity.
(2-5) The difference between the maximum value of the aforesaid mi and its minimum value and the difference between the maximum value of the aforesaid ni and its minimum value are both 2 or larger.
(2-6) The aforesaid k is equal to 4, and when xcfx86max denotes the diameter of the aperture stop at the time of using the aforesaid first optical disk, hi denotes the height from the aforesaid step portion, the border of the ring-shaped bands, which divides the light flux into the first light flux and the second light flux, to the optical axis, and h3 denotes the height from the aforesaid step portion, the border of the ring-shaped bands, which divides the light flux into the third light flux and the fourth light flux, to the optical axis, these satisfy the following conditions:
0.62xe2x89xa6h1/0.5xcfx86maxxe2x89xa60.69xe2x80x83xe2x80x83(3), 
and
0.70xe2x89xa6h3/0.5xcfx86maxxe2x89xa60.80xe2x80x83xe2x80x83(4). 
If h1/0.5xcfx86max is smaller than the lower limit value, the spot light intensity becomes low in the first optical disk, and on the contrary, if it exceeds the upper limit value, the wave front aberration of the first light flux is degraded in the second optical disk, which lowers the central intensity of the spot light. Further, if h3/0.5xcfx86max is smaller than the lower limit value, the spot size does not meet the required value in the second optical disk, and on the contrary, if it exceeds the upper limit value, the specification becomes excessive for the second optical disk, and as the bad influence of it, the spot light quantity at the time of using the first optical disk becomes insufficient.
(2-7) The aforesaid k is equal to 6, and the second light flux and the fifth light flux form the apparent best image plane at a position different from the position of the best image plane formed by the sixth light flux.
(2-8) The aforesaid k is equal to 6, and when xcfx86max denotes the diameter of the aperture stop at the time of using the first optical disk, hl denotes the height from the aforesaid step portion, the border of the ring-shaped bands, which divides the light flux into the first light flux and the second light flux, to the optical axis, and h5 denotes the height from the aforesaid step portion, the border of the ring-shaped bands, which divides the light flux into the fifth light flux and the sixth light flux, to the optical axis, these satisfy the following conditions:
0.52xe2x89xa6h1/0.5xcfx86maxxe2x89xa60.67xe2x80x83xe2x80x83(5), 
and
0.70xe2x89xa6h5/0.5xcfx86maxxe2x89xa60.82xe2x80x83xe2x80x83(6). 
If h1/0.5xcfx86max is smaller than the lower limit value, the central intensity of the spot light becomes low in the first optical disk, and on the contrary, if it exceeds the upper limit value, the wave front aberration of the first light flux is degraded in the second optical disk, which lowers the central intensity of the spot light. Further, if h5/0.5xcfx86max is smaller than the lower limit value, the spot size does not meet the required value in the second optical disk, and on the contrary, if it exceeds the upper limit value, the specification becomes excessive for the second optical disk, and as the bad influence of it, the spot light quantity at the time of using the first optical disk becomes insufficient.
(2-9) An optical pickup device for recording and/or reproducing information which comprises a converging optical system including an objective lens for converging light flux from light sources having different wavelengths respectively on the recording surface of an optical disk and a light receiving means for detecting reflected light from said recording surface and is capable of recording and/or reproducing information for a first and a second optical disks having transparent substrates of different thicknesses respectively (wherein the required numerical aperture of the objective lens for the first optical disk is supposed to be larger than that for the second optical disk),
wherein ring-shaped divisional surfaces for dividing the incident light flux by refracting action into k (kxe2x89xa74) pieces of ring-shaped light flux (where the light flux are called consecutively the first, the second, - - - , and the k-th light flux from the optical axis side toward the outer side) are formed on at least one of the surfaces of said objective lens,
when said first optical disk is used, the spherical aberration component of the wave front aberration of said first and k-th light flux at the position of the best image plane formed by said first and k-th light flux is 0.05xcex1 rms or under (xcex1: the wavelength of the light source for the first optical disk),
for at least two pieces of light flux among said second to (kxe2x88x921)-th light flux, the apparent position of the best image plane comes to a position different from said position of the best image position formed by said first and k-th light flux, and
at said position of the best image plane formed by said first and k-th light flux, the wave front aberration of the rays in each of said first to k-th light flux passing the inside of the required numerical aperture for said first optical disk becomes approximately mixcex1 (mi is an integer, where i=1, 2, - - - , k).
According to the optical pickup device described in the structure (2-9), because its objective lens is designed to make the residual aberration small for the thickness of the substrate falling within the range from the substrate thickness of said first optical disk to the substrate thickness of said second optical disk-the recording and/or reproducing of information can be suitably performed for plural kinds of optical disks.
(2-10) In the objective lens, when the aforesaid first optical disk is used, at the aforesaid position of the best image plane formed by the aforesaid first and k-th light flux, the amount of the wave front aberration w1 of the rays in each of said first to k-th light flux satisfies the following condition:
(mixe2x88x920.30)xcex1xe2x89xa6w1xe2x89xa6(mi+0.30)xcex1xe2x80x83xe2x80x83(1) 
For example, in the case where a DVD is used for the optical disk, if w1 does not satisfy the above expression (1), deviation of the wave front aberration is produced, which lowers the spot light intensity.
(2-11) In the objective lens, in carrying out the recording or reproducing of information by using a light flux from the light source having a wavelength of xcex2 for the aforesaid second optical disk-the wave front aberration of the rays in the first to (kxe2x88x921)th light flux becomes approximately nixcex2 (ni is an integer, where i=1, 2, - - - , kxe2x88x921). Accordingly, in said second optical disk-the residual aberration can be made small, and owing to it, the recording and/or reproducing of information can be carried out for said second optical disk.
(2-12) In the objective lens, in carrying out the recording or reproducing of information by using a light flux from the light source having a wavelength of xcex2 for the aforesaid second optical disk-the amount of the wave front aberration w2 of the rays in each of the said first to (kxe2x88x921)th light flux satisfies the following condition in the required numerical number for said second optical disk:
(nixe2x88x920.30)xcex2xe2x89xa6w2xe2x89xa6(ni+0.30)xcex2xe2x80x83xe2x80x83(2). 
For example, in the case where a CD is used for the optical disk, if w2 does not satisfy the above expression (2), deviation of the wave front aberration is produced, which lowers the spot light intensity.
(2-13) In the objective lens, the difference between the maximum value of the aforesaid mi and its minimum value and the difference between the maximum value of the aforesaid ni and its minimum value are both 2 or larger.
(2-14) In the objective lens, the aforesaid k is equal to 4, and when xcfx86max denotes the diameter of the aperture stop at the time of using the aforesaid first optical disk, h1 denotes the height from the aforesaid step portion, the border of the ring-shaped bands, which divides the light flux into the first light flux and the second light flux, to the optical axis, and h3 denotes the height from the aforesaid step portion, the border of the ring-shaped bands, which divides the light flux into the third light flux and the fourth light flux, to the optical axis, these satisfy the following conditions:
0.62xe2x89xa6h1/0.5xcfx86maxxe2x89xa60.69xe2x80x83xe2x80x83(3), 
and
0.70xe2x89xa6h3/0.5xcfx86maxxe2x89xa60.80xe2x80x83xe2x80x83(4). 
If h1/0.5xcfx86max is smaller than the lower limit value, the spot light intensity becomes low in the first optical disk, and on the contrary, if it exceeds the upper limit value, the wave front aberration of the first light flux is degraded in the second optical disk, which lowers the central intensity of the spot light. Further, if h3/0.5xcfx86max is smaller than the lower limit value, the spot size does not meet the required value in the second optical disk, and on the contrary, if it exceeds the upper limit value, the specification becomes excessive for the second optical disk, and as the bad influence of it, the spot light quantity at the time of using the first optical disk becomes insufficient.
(2-15) In the objective lens, the aforesaid k is equal to 6, and the second light flux and the fifth light flux form the apparent best image plane at a position different from the position of the best image plane formed by the sixth light flux.
(2-16) In the objective lens, the aforesaid k is equal to 6, and when xcfx86max denotes the diameter of the aperture stop at the time of using the first optical disk, h1 denotes the height from the aforesaid step portion, the border of the ring-shaped bands, which divides the light flux into the first light flux and the second light flux, to the optical axis, and h5 denotes the height from the aforesaid step portion, the border of the ring-shaped bands, which divides the light flux into the fifth light flux and the sixth light flux, to the optical axis, these satisfy the following conditions:
0.52xe2x89xa6h1/0.5xcfx86maxxe2x89xa60.67xe2x80x83xe2x80x83(5), 
and
0.70xe2x89xa6h5/0.5xcfx86maxxe2x89xa60.82xe2x80x83xe2x80x83(6). 
If h1/0.5xcfx86max is smaller than the lower limit value, the central intensity of the spot light becomes low in the first optical disk, and on the contrary, if it exceeds the upper limit value, the wave front aberration of the first light flux is degraded in the second optical disk, which lowers the central intensity of the spot light. Further, if h5/0.5xcfx86max is smaller than the lower limit value, the spot size does not meet the required value in the second optical disk, and on the contrary, if it exceeds the upper limit value, the specification becomes excessive in the second optical disk, and as the bad influence of it, the spot light quantity at the time of using the first optical disk becomes insufficient.
In this specification, xe2x80x9can objective lensxe2x80x9d in a narrow sense means a lens which has a light converging function and is arranged at the nearest position to the optical information recording medium opposite to it, and in a broad sense, it means a lens which is arranged at a position opposite to the optical information recording medium and is capable of moving in the direction of the optical axis by an actuator.
In this specification, as for the optical information recording medium (optical disk), not only present disk-shaped optical information recording media, for example, various kinds of CD""s such as a CD, a CD-R, a CD-RW, a CD-Video, and a CD-ROM, various kinds of DVD""s such as a DVD, a DVD-ROM, a DVD-RAM, a DVD-R, and a DVD-RW, and an MD, but also a recording medium of the next generation is included. On the information recording surface of most of the optical information recording media, a transparent substrate is present.
In this specification, xe2x80x9crecording and reproducing of informationxe2x80x9d means recording information on the information recording surface of such an information recording medium as the above-mentioned, and reproducing information recorded on the information recording surface of the same. An optical system of this invention may be such one as used for carrying out recording only or reproducing only, or may be such one as used for carrying out both of recording and reproducing. Further, it may be such one as used for recording for a certain information recording medium and used for carrying out reproducing for some other information recording medium, or may be such one as used for carrying out recording or reproducing for a certain information recording medium and carrying out recording and reproducing for some other information recording medium. In addition, xe2x80x9creproducingxe2x80x9d as mentioned herein includes merely reading information.
xe2x80x9cA required numerical aperturexe2x80x9d in this application desirably means a numerical aperture which is required for enabling reproducing or recording of information on an optical information recording medium with a light flux having a specified wavelength in an optical pickup device, but it is also possible to regard a numerical aperture which is determined by a standard of each of optical information recording media as a required numerical aperture.
An optical pickup device of this invention can be provided in a player or a drive which is compatible to optical information recording media such as a CD, a CD-R, a CD-RW, a CD-Video, a CD-ROM, a DVD, a DVD-ROM, a DVD-RAM, a DVD-R, a DVD-RW, and an MD, or in a recording and/or reproducing apparatus of audio and/or image such as an AV apparatus, a personal computer, and other information terminals which have the above-mentioned player or drive built in.